Most cutting tools for turning, milling, drilling or other chip forming machining are today coated with a wear resistant coating deposited using chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques in order to prolong the service life of the tool and/or to increase the productivity. In general a comparatively thick coating is aimed for since this improves the wear resistance. This is readily accomplished for CVD coatings. However, PVD coatings have several attractive properties compared to CVD coatings, in particular the ability to provide compressive stress in the coating which gives improved toughness as compared to the CVD coatings. These compressive stresses essentially originate from differences in thermal expansion between substrate and coating and a densification effect due to a bombardment of high energy ions during deposition. This advantageous effect of ion bombardment is hardly present in all PVD techniques such as thermal evaporation, but prevails in sputter deposition and with cathodic arc deposition the compressive stress may become excessively high, often greater than 5 GPa, due to high kinetic energy of impinging ions. Biasing of the substrate is a standard way of increasing the kinetic energy and is for hard coatings considered necessary to obtain desired mechanical properties. The densification typically leads to increased hardness and improved wear resistance of the coating material. In combination with a potentially high deposition rate this makes the cathodic arc deposition techniques an interesting alternative for deposition of coatings for cutting tools.
However, due to too heavy ion bombardment defects may be generated and too high compressive stresses will eventually cause delamination of the coating, spontaneously or due to forces acting on the coating during machining, in particular for thick coatings. Thus the stress level in the coating is normally controlled to obtain highest possible compressive stress without impairing the adhesion of the coating too much. In particular flaking along the edge of a coated cutting tool insert is a well known problem. For example, U.S. Pat. No. 7,838,132 discloses cathodic arc deposition of about 3 μm thick (Ti,Al)N coatings on cemented carbide substrates using an arc current of 200 A at different substrate bias ranging from −40 V to −200 V and concludes that there is an optimum at about −70 V where high compressive stresses, i.e. more than 4 GPa, and still good adhesion can be achieved.
Consequently, prior art cathodic arc deposition processes suffer from a trade-off between coating thickness, which is crucial for wear resistance, and quality of the coating with respect to mechanical properties, in particular toughness.